There is a continuing effort in the retroreflective industry to produce products of greater retroreflectivity. The present invention achieves such an increase in retroreflectivity by discovery of transparent glass microspheres that have improved optical, physical, and chemical properties.
The superiority of the new microspheres can be seen by comparing them with the microspheres used in products that presently achieve industry-setting standards of retroreflectivity. One deficiency in the prior-art microspheres is that under normal storage conditions they tend to clump together, probaby because they absorb moisture. This clumping can reduce the uniformity and density of distribrtion of the microspheres in a retroreflective product, and the result is a limitation on the intensity of retroreflection from the product.
Another fault of the prior-art microspheres is that they lack stability in composition. Thus, the composition of the microspheres, and accordingly their properties, varies with small, difficult-to-avoid changes in processing parameters. One important variation is a variation throughout a batch of the prior-art microspheres as to index of refraction, which determines the retroreflective properties of the microspheres and which is a design parameter for retroreflective products that include them. The larger the diameter of the microspheres in the batch, the lower their index of refraction. It is theorized that this difference an index of refraction occurs during formation of the microspheres, as a result of a tendency for smaller microspheres to lose easily-vaporized low-index ingredients.
Since maximum retroreflectivity is obtained in a retroreflective product when all the microspheres in the product have the same predetermined index of refraction (in order to obtain precise focusing of incident light rays on a reflective surface disposed at the back of the microspheres; for many retroreflective sheeting products, an index of refraction of 1.93 is optimum), the lack of uniformity in index of refraction reduces retroreflectivity.
Other important consequences of instability in composition are wastage, because microspheres are formed that do not satisfy established specifications; and a need for more extensive process controls, which increases manufacturing costs.
In summary, while the prior-art microspheres have provided very useful retroreflective products, improvement is desired and can lead to advances in the retroreflective industry.